Thermostatic fiber optic waveguides

Abstract

Fiber optic waveguides exhibiting a blackout phenomenon can be used for temperature sensing. A temperature sensing waveguide can be used in such applications as maintaining a material within a selected temperature range, freeze protection, viscosity control of liquids in pipelines, leak detection of cryogenic fluids, fire detection, application of heat-recoverable materials, and fluid level detection. Novel waveguides exhibiting blackout at selected temperatures for use in these applications are described. Among the novel waveguides are those having a cladding comprising a polyalkylphenyl siloxane and those having a cladding comprising a cross-linked polymethylalkyl siloxane. Also described are waveguides where only a section of the waveguide exhibits blackout and methods for making such waveguides.

What is claimed is: 
     
       1. A waveguide comprising two connected sections, a first section and a second section, the first section comprising a first solid core and a first solid cladding disposed on and around the exterior surface of the first core, and the second section comprising a second solid core and a second solid cladding disposed on and around the exterior surface of the second core, the first section of the waveguide exhibiting blackout at a first temperature T 1 , the second section being capable of transmitting light at T 1 . 
     
     
       2. The waveguide of claim 1 in which the first core and the second core are of the same material. 
     
     
       3. The waveguide of claim 1 or 2 in which the second section of the waveguide exhibits blackout at a second temperature T 2 , the first section being capable of transmitting light at T 2 . 
     
     
       4. The waveguide of claim 1 or 2 including a third section comprising a third core and a third cladding disposed on and around the exterior surface of the third core, the third section of the waveguide exhibiting blackout at a third temperature T 3 , the first and second sections being capable of transmitting light at T 3 . 
     
     
       5. The waveguide of claim 4 in which the first, second and third cores are all of the same material. 
     
     
       6. A waveguide comprising a plurality of sensing elements separated from each other by light transmitting elements, each sensing element comprising a solid sensing core and a solid sensing cladding disposed on and around the exterior surface of the sensing core, and each light transmitting element comprising a solid transmitting core and a solid transmitting cladding disposed on and around the exterior surface of the transmitting core, each sensing element exhibiting blackout at a temperature at which the light transmitting elements are capable of transmitting light. 
     
     
       7. The waveguide of claim 6 in which the cores of the sensing elements and the cores of the light transmitting elements are of the same material. 
     
     
       8. The waveguide of claim 6 in which the core of at least one sensing element and the core of at least one light transmitting element are of the same material. 
     
     
       9. The waveguide of claim 6 comprising two sensing elements. 
     
     
       10. The waveguide of claim 6 in which at least one sensing element has a different blackout temperature than another sensing element. 
     
     
       11. The waveguide of claim 6 in which all the sensing elements have cores of the same material. 
     
     
       12. The waveguide of claim 6 or 11 in which the light transmitting elements have cores of the same material. 
     
     
       13. A waveguide comprising a solid core and a first section of solid cladding disposed on and around the exterior surface of the core, and a second section of solid cladding disposed on and around the exterior surface of the core, the first section and the second section being of different compositions. 
     
     
       14. The waveguide of claim 13 in which the refractive indices of both sections of cladding are less than the refractive index of the core at all temperatures less than a temperature T 1 , and at temperatures greater than T 1  the refractive index of the first section of cladding is greater than the refractive index of the core while the refractive index of the second section of cladding remains less than the refractive index of the core. 
     
     
       15. The waveguide of claim 13 in which the refractive indices of both sections of cladding are less than the refractive index of the core at all temperatures greater than a temperature T 1 , and at temperatures less than T 1 , the refractive index of the first section of cladding is greater than the refractive index of the core while the refractive index of the second section of cladding remains less than the refractive index of the core. 
     
     
       16. A method for preparing a waveguide comprising a core and a first section of solid cladding disposed on and around the exterior surface of the core and a second section of solid cladding disposed on and around the exterior surface of the core, the first section comprising a first material and the second section comprising a second material, the first and second materials being different, the method comprising the steps of: (a) fabricating a waveguide comprising a solid core and a first solid cladding comprising the first material;   (b) removing a section of the first solid cladding; and   (c) curing the second solid cladding in situ on the section of the core with the first cladding removed therefrom.   
     
     
       17. A method for preparing a waveguide comprising a solid core and a first section of solid cladding disposed on and around the exterior surface of the core and a second section of solid cladding disposed on and around the exterior surface of the core, the first section comprising a first material and the second section comprising a second material, the first and second materials being different, the method comprising the steps of: (a) fabricating a waveguide comprising a solid coare and a first solid cladding comprising the first material;   (b) removing a section of the first solid cladding; and   (c) heat shrinking a heat-recoverable sleeve onto the section of the core with the first cladding removed therefrom, the sleeve having an insert of the second solid cladding.   
     
     
       18. A method for preparing a waveguide comprising a solid core and a first section of solid cladding disposed on and around the exterior surface of the core and a second section of solid cladding disposed on and around the exterior surface of the core, the first section comprising a first material and the second section comprising a second material, the first and second materials being different, the method comprising the steps of: (a) fabricating a waveguide comprising a solid core and a first solid cladding comprising the first material; and   (b) doping a section of the first solid cladding with a dopant that alters the refractive index of the first cladding, thereby forming the second material.   
     
     
       19. A method for detecting fires comprising the steps of: (a) placing at least part of a waveguide in a position proximate to a combustible material so that the temperature of the waveguide is higher than ambient temperature when the combustible material is on fire, the waveguide comprising a fiber core and a cladding disposed on and around the exterior surface of the core, the waveguide exhibiting blackout when the temperature of said part is at a blackout temperature that is higher tham ambient temperature but less than 200° C.;   (b) directing light at an end of the waveguide;   (c) monitoring the intensity of light transmitted by the waveguide, the onset of a substantial change in intensity indicating that the combustible material is on fire; and   (d) stopping the fire.   
     
     
       20. A waveguide comprising a solid core and a solid cladding disposed on and around the exterior surface of the core, at least a portion of the waveguide exhibiting blackout at a blackout temperature greater than about -20° C. and less than about 200° C. 
     
     
       21. The waveguide of claim 20 wherein said portion exhibits blackout at about 0° C. 
     
     
       22. The waveguide of claim 20 or 21 wherein said portion exhibits blackout at a temperature less than about 100° C. 
     
     
       23. The waveguide of claim 20 wherein at temperatures greater than the blackout temperature, said portion is capable of transmitting light without substantial attenuation and at temperatures less than the blackout temperature, said portion is not capable of transmitting light. 
     
     
       24. The waveguide of claim 20 wherein at temperatures less than the blackout temperature, said portion is capable of transmitting light without substantial attenuation and at temperatures greater than the blackout temperature, said portion is not capable of transmitting light. 
     
     
       25. The waveguide of claim 20 wherein at least a portion of the waveguide is capable of transmitting light with a first attenuation at temperatures less than the blackout temperature and is capable of transmitting light with a second attentuation at temperatures greater than the blackout temperature, the first and second attenuation differing by at least about 3 dB. 
     
     
       26. The waveguide of claim 20 wherein at least two portions of the waveguide exhibit blackout at a blackout temperature greater than about -20° C. and less than about 200° C., said portions being separated by a portion that does not exhibit blackout at the selected temperature. 
     
     
       27. A waveguide comprising a solid core and a solid cladding disposed on and around the core, the index of refraction of the cladding being greater than or equal to the index of refraction of the core at temperatures of T 1  and less, T 1  being greater than about -20° C., and less than about 200° C., the index of refraction of the cladding being less than the index of refraction of the core at temperatures greater than T 1 . 
     
     
       28. The waveguide of claim 27 wherein T 1  is about 0° C. 
     
     
       29. The waveguide of claim 27 wherein T 1  is greater than 0° C. 
     
     
       30. A waveguide comprising a core and cladding disposed thereon, the cladding having a crystalline melting point greater than -20° C., the refractive index of the cladding being less than the refractive index of the core at temperatures greater than the crystalline melting point of the cladding and being greater than or equal to the refractive index of the core at temperatures less than or equal to the crystalline melting point. 
     
     
       31. The waveguide of claim 30 in which the cladding comprises a copolymer of dimethylsiloxane and ethylene oxide. 
     
     
       32. The waveguide of claim 30 in which the cladding comprises a cross-linked polydialkyl siloxane. 
     
     
       33. A waveguide comprising a core and cladding disposed on and around the exterior surface of the core, the cladding comprising a polyalkylphenyl siloxane, the refractive index of the cladding being less than the refractive index of the core at temperatures greater than a temperature T 1 , the polyalkylphenyl siloxane containing sufficient phenyl that at temperatures less than T 1 , the refractive index of the cladding is greater than the refractive index of the core so that the waveguide exhibits blackout at T 1  and is capable of transmitting light without substantial attenuation at temperatures greater than T 1 , wherein T 1  is no higher than 15° C. 
     
     
       34. The waveguide of claim 33 in which the cladding comprises a polymethylphenyl siloxane. 
     
     
       35. The waveguide of claim 34 in which the phenyl content of the polymethylphenyl siloxane is at least about 15% by weight based upon the total weight of the siloxane. 
     
     
       36. The waveguide of claim 33 in which the cladding comprises a blend of two methylphenyl siloxanes of different phenyl content, the refractive index of the two methylphenyl siloxanes being different by no more than 0.02 and the viscosity of both methylphenyl siloxanes being in the range of 500 to 10,000 cps. 
     
     
       37. The waveguide of claim 33 wherein T 1  is 10° C. 
     
     
       38. The waveguide of claim 33 wherein T 1  is 5° C. 
     
     
       39. The waveguide of claim 33 wherein T 1  is 0° C. 
     
     
       40. A waveguide comprising a fiber core and a cladding disposed on and around the exterior surface of the core, the cladding having a crystalline melting point greater than -20° C., the refractive index of the cladding being less than the refractive index of the core at temperatures greater than the crystalline melting point of the cladding and being greater than or equal to the refractive index of the core at temperatures less than or equal to the crystalline melting point, the cladding comprising a cross-linked polydialkyl siloxane comprising the unit: ##STR2## where each R 1  in a polymer chain is independently selected from the group consisting of methyl, ethyl, and propyl groups; and where each R 2  in a polymer chain is independently an alkyl group of at least 10 carbon atoms and the R 1  's in a polymer chain can be the same or different and the R 2  's in a polymer chain can be the same or different.   
     
     
       41. The waveguide of claim 40 in which each R 1  is a methyl group. 
     
     
       42. The waveguide of claim 40 or 41 wherein each R 2  is tetradecane. 
     
     
       43. The waveguide of claim 40 or 41 wherein a portion of the R 2  's are tetradecane. 
     
     
       44. The waveguide of claim 40 wherein each R 2  is a linear alkyl group. 
     
     
       45. A waveguide comprising a solid core and a solid cladding disposed on and around the core, the index of refraction of the cladding being greater than or equal to the index of refraction of the core at temperatures of T 1  and greater, T 1  being greater than about -20° C., and less than about 200° C., the index of refraction of the cladding being less than the index of refraction of the core at temperatures less than T 1 . 
     
     
       46. The waveguide of claim 45 wherein T 1  is about 0° C. 
     
     
       47. The waveguide of claim 45 wherein T 1  is greater than 0° C. 
     
     
       48. A waveguide comprising a solid core and solid cladding disposed thereon, the core having a crystalline melting point greater than -20° C., the refractive index of the core being less than the refractive index of the cladding at temperatures greater than the crystalline melting point of the core and being greater than or equal to the refractive index of the cladding at temperatures less than or equal to the crystalline melting point. 
     
     
       49. A waveguide comprising a fiber core and a cladding disposed on and around the exterior surface of the core, the core having a crystalline melting point greater than -20° C., the refractive index of the core being less than the refractive index of the cladding at temperatures greater than the crystalline melting point of the core and being greater than or equal to the refractive index of the cladding at temperatures less than or equal to the crystalline melting point, the core comprising a cross-linked polydialkyl siloxane comprising the unit: ##STR3## where each R 1  in a polymer chain is independently selected from the group consisting of methyl, ethyl, and propyl groups; and where each R 2  in a polymer chain is independently an alkyl group of at least 10 carbon atoms and the R 1  's and R 2  's in a polymer chain can be the same or different.   
     
     
       50. The waveguide of claim 49 in which each R 1  is a methyl group. 
     
     
       51. The waveguide of claim 49 wherein each R 2  is tetradecane. 
     
     
       52. The waveguide of claim 49 wherein a portion of the R 2  's are tetradecane. 
     
     
       53. The waveguide of claim 49 wherein each R 2  is a linear alkyl group. 
     
     
       54. A waveguide comprising a core, a first light transmissive cladding disposed on and around the exterior surface of the core, and a second cladding disposed on and around the exterior surface of the first cladding, the refractive index of the first cladding being less than the refractive index of the core at temperatures less than a first temperature, T 1 , and being greater than or equal to the refractive index of the core at temperatures greater than T 1 , the refractive index of the second cladding being less than the refractive index of the first cladding at temperatures less than a second temperature, T 2 , and greater than or equal to the refractive index of the first cladding at temperatures greater than T 2 , T 2  being greater than T 1 , wherein at temperatures less than T 1  the waveguide is capable of transmitting light through the core, and at temperatures less than T 2  and greater than T 1 , the waveguide is capable of transmitting light through the first cladding, and at temperatures greater than T 2 , substantially no light is transmitted through the core and the first cladding. 
     
     
       55. The waveguide of claim 54 wherein the thickness of the first cladding is less than the thickness of the second cladding. 
     
     
       56. A waveguide comprising a core and cladding disposed thereon, the cladding having a crystalline melting point of about 1° C., the refractive index of the cladding being less than the refractive index of the core at temperatures greater than the crystalline melting point of the cladding and being greater than or equal to the refractive index of the core at temperatures less than or equal to the crystalline melting point. 
     
     
       57. A waveguide comprising a fiber core and a cladding disposed on and around the exterior surface of the core, the cladding having a crystalline melting point greater than -20° C., the refractive index of the cladding being less than the refractive index of the core at temperatures greater than the crystalline melting point of the cladding and being greater than or equal to the refractive index of the core at temperatures less than or equal to the crystalline melting point, the cladding comprising a cross-linked polydialkyl siloxane comprising the unit: ##STR4## where each R1 in a polymer chain is independently selected from the group consisting of methyl, ethyl, and propyl groups; and where a portion of the R2's are independently alkyl groups of at least 10 carbon atoms and a portion of the R2's are fluorine substituted groups,   wherein the R1's in a polymer chain can be the same or different.   
     
     
       58. The waveguide of claim 57 wherein a portion of the R2 groups are tri-fluoro propyl. 
     
     
       59. A waveguide comprising a fiber core and a cladding disposed on and around the exterior surface of the core, the cladding having a crystalline melting point greater than -20° C., the refractive index of the cladding being less than the refractive index of the core at temperatures greater than the crystalline melting point of the cladding and being greater than or equal to the refractive index of the core at temperatures less than or equal to the crystalline melting point, the cladding comprising a cross-linked polydialkyl siloxane comprising the unit: ##STR5## where a portion of the R's are alkyl groups of at least 10 carbon atoms and a portion of the R's are trifluoropropyl.